Jan his tool is not going to work, it depends on the BIOS. The only way to change the brightness to try and change the brightness when before ArcaOS is loaded (BIOS screen is displayed). But the keys might not even work anymore. Some of the older laptops have this functionality build in the BIOS. But when ACPI.PSD loads these options get disabled.

At Warpstock Europe in Berlin David from Arca Noae LLC explained the following this switch might work. The INF file explains the following:

/NOACPI Prevents the PSD from enabling some ACPI services. This switch is experimental.

WARNING: There is absolutely NO SUPPORT for the use of this switch. Your use of this switch is completely at your own risk. There is no guarantee that it will work, or that your system will even run when using this switch. There is no support for anything for any reason if you use this switch. If you are using this switch and you open a ticket, you may need to remove the switch and retest before you can get support.
(end of copy paste).

Before you use this switch behind psd=acpi.psd make certain you can boot from DVD to take the switch out the config.sys.